The storage of heat in compact inexpensive form is becoming more and more necessary both in the utilization of solar energy and in the leveling of electrical daily peak loads. While much of this interest is focused on low or medium temperature storage where rocks, water tanks or the latent heat of fusion of salt hydrates may be used, there is also a need for high temperature storage, for example such as in the generation of electricity by thermal processes from the heat of the sun by focussing reflectors, for heat energy storage for use with fossil-fueled power boilers to generate steam during peak periods, and in the use of electric resistance heat for off-peak space heating, and other applications as will be explained further below.
In Europe there has been great success in the use of magnesite brick (magnesium carbonate) in off-peak space heating using individual room heaters, but magnesite brick has been too expensive for larger uses, and the relatively large bricks are slow to heat and cool due to poor heat transfer, thereby limiting the rate at which heat can be stored and withdrawn in practical equipment. These multiple heaters are inherently expensive.
The use of anhydrous sodium sulfate salt compressed to high density in bars, slabs or bricks was disclosed in my U.S. Pat. No. 2,949,679. Aluminum foil was used by me to wrap the bars both to provide heat transfer and to prevent dusting and crumbling of the sodium sulfate bars. Later it was found that an explosive situation may occur if the aluminum foil should melt when touching the hot salt, the aluminum foil having a melting point at approximately 1200.degree. F.
As explained in the above patent, sodium sulfate exhibits dimorphism and has a very high sensible heat content as well as a dimorphic latent heat of crystal phase transformation at about 460.degree.-470.degree. F. However, in addition to the latent heat, the apparatus described in that patent only used the sensible heat of the aluminum-foil-wrapped bars, slabs or bricks up to about 500.degree.-600.degree. F.
As will be explained further below, the present invention enables the high specific heat and latent heat of anhydrous sodium sulfate salt to be utilized practicably up to a temperature level nearing the salts melting point which occurs above 1600.degree. F.
When compared with magnesite brick the total heat content per cubic foot of solid sodium sulfate is twenty-five percent greater up to the usual magnesite brick practical maximum temperature of 1500.degree. F., and the cost of anhydrous sodium sulfate is one-quarter of magnesite or even less for by-product impure sodium sulfate, such as chrome cake, a by-product of chromium manufacture. In the range of 400.degree. to 600.degree. F. the heat content is seventy-two percent greater when compared to magnesite brick and about twenty-three percent greater compared to rock.